The present invention relates to methods and apparatus for positioning ophthalmic lenses for grinding and, in particular, for automatically situating a lens blank relative to a grinding tool.
A traditional technique for grinding ophthalmic lenses involves making repeated grinding passes across a lens blank by means of a rotary grinding cup until a required amount of the blank has been ground away, leaving a lens with a desired center thickness. The amount of the blank to be ground away can be calculated by subtracting the desired final thickness from original thickness.
In an effort to avoid the need for making calculations, a practice which is susceptible of error, a lens positioning technique was proposed in Coburn et al U.S. Pat. No. 3,289,355, issued Dec. 6, 1966 in which the positioning of a lens was achieved by the setting of a pair of dials in a manner avoiding the need for making calculations. In particular, that patent discloses a tool supporting assembly disposed opposite a tool grinding mechanism including a rotary grinding cup. The grinding mechanism is adjustable to properly orient the grinding cup for grinding a desired base curve and cross curve. The lens supporting mechanism is displaceable toward and away from the grinding mechanism and includes a stop for engaging the housing of the grinding mechanism. The stop is adjustable in location relative to the lens by means of a first dial which is calibrated by values corresponding to known lens diopter such that the stop is adjusted in accordance with the sagitta value of the lens, i.e., that portion of the lens thickness extending between the lens front curve and a lens reference plane defined by the lens holder. As a result of such an adjustment of the stop, the lens supporting mechanism can be initially displaced forwardly toward the grinding mechanism until the stop engages a cooperating abutment on the housing of the lens grinding mechanism, whereupon the lens reference plane will lie forwardly of a tool reference axis of the grinding mechanism by a distance equal to the sagitta value (S). The operator then sets a second dial which is calibrated in accordance with desired lens thickness to displace the lens holder rearwardly relative to the stop arm by a distance equal to the desired lens thickness (T). This dial adjustment is performed in step-by-step fashion following grinding sweeps of the tool. At the end of the movement of the second dial, i.e., when the dial reaches the desired thickness value, the lens reference plane will be spaced rearwardly of the tool reference axis by a distance equal to T-S which distance represents the amount of the lens which has been removed by the grinding tool in order to achieve the desired lens thickness.
In order for such a mechanism to produce sufficiently accurate lenses, it is necessary that the lens reference plane be precisely positioned relative to the tool reference axis at the end of the initial forward advancement of the lens supporting mechanism wherein the stop contacts the tool supporting mechanism. However, such precise positioning would be difficult to achieve in that system for a number of reasons. For example, the stop is disclosed as comprising at least three pivotably interconnected components, whereby there would be a considerable likelihood of relative play occurring between the components, and hence a positional inaccuracy of the stop.
Also, it is not uncommon for the lens supporting mechanism to rebound slightly when the stop engages the tool supporting mechanism, thereby creating a slight gap between the stop and the tool supporting mechanism. As a result, the location of the lens reference plane relative to the tool reference axis would be offset by a distance equal to such a gap, unbeknownst to the operator, and inaccuracies would occur in the grinding of the lens.
It would be desirable to enable a lens to be accurately ground to a desired thickness without the need for an operator to make calculations in advance of the grinding operation, or to take measurements of the lens during the grinding operation. It would also be desirable to enable a lens to be ground with minimal chance of dimensional inaccuracies occurring.